From The University of Tokyo [(東京大学](JP)
6.13.24
An international team of astronomers, led by Yasunori Hori and Teruyuki Hirano from Astrobiology Center and Akihiko Fukui and Norio Narita from The University of Tokyo, has reported the discovery and follow-up of four short-period mini-Neptunes around red dwarfs older than one billion years.
Diagram of discovered exoplanet orbits. The orbits of exoplanets close to their parent stars tend to become circular over time, but three of the newly discovered exoplanets, except the bottom left, have maintained elliptical orbits despite being over a billion years old. (credit: Astrobiology Center)
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SUMMARY
Our team discovered mini-Neptunes*1 around four red dwarfs*2, which are named TOI-782, TOI-1448, TOI-2120, and TOI-2406, using observations from a global network of ground-based telescopes with MuSCATs and the TESS space telescope*3.
These four mini-Neptunes are close to their parent stars, and the three of them are likely to be in eccentric orbits (TOI-782 b, TOI-2120 b, TOI-2406 b).
These mini-Neptunes are not rocky planets like Earth but may be Neptune-like planets.
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At least three of these mini-Neptunes are likely to be in eccentric orbits. The fact that these mini-Neptunes have maintained non-zero eccentricities for billions of years after their birth suggests that they may not be rocky planets like Earth but Neptune-like planets that are less susceptible to tidal deformation. This study should provide a clue to the origins and elusive interior structures of mini-Neptunes.
This paper was published in The Astronomical Journal on May 30, 2024.
INTRODUCTION
Planets between the size of Earth and Uranus/Neptune, known as mini-Neptunes, are not found in our Solar System. However, mini-Neptunes are relatively common outside the Solar System and are promising targets for atmospheric characterization by Webb. What do mini-Neptunes look like?
RESULTS
We have discovered four transiting*4 short-period mini-Neptunes orbiting red dwarfs (TOI-782, TOI-1448, TOI-2120, and TOI-2406) through follow-up observations with ground-based telescopes with MuSCATs (a series of Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets*5). These mini-Neptunes have radii about 2-3 times that of Earth and orbital periods of less than eight days. In addition, our radial velocity measurements*6 of their parent stars, obtained with the IRD (InfraRed Doppler) on the Subaru telescope, indicate that the upper limit on the masses of these four planets is less than 20 times the mass of Earth. The relationship between the measured radii and the upper mass limits of these mini-Neptunes suggests that they are not rocky planets like Earth. Their interiors likely contain volatiles such as icy materials like H2O and atmospheres.
We also found that at least three of these four mini-Neptunes (TOI-782 b, TOI-2120 b, TOI-2406 b) are likely to be in eccentric orbits. In general, the orbit of a short-period planet around a red dwarf should be circular due to tidal dissipation. However, three short-period mini-Neptunes around red dwarfs have maintained non-zero eccentricities for billions of years. One possible interpretation of this is that their interiors are not susceptible to tidal effects. The mass-radius relationship of these four mini-Neptunes suggests that they are not rocky planets. Thus, the interiors of these mysterious mini-Neptunes may be similar to those of Neptune. Short-period mini-Neptunes are promising targets for atmospheric observations with the James Webb Space Telescope. Further detailed follow-up observations are expected to improve our understanding of the internal compositions and atmospheres of short-period mini-Neptunes.
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*1: Mini-Neptunes or sub-Neptunes are planets between the size of Earth and Neptune (about 4 times the radius of Earth).
*2: M-type stars with effective temperatures below ~3,800K.
*3: NASA’s space telescope, the Transiting Exoplanet Survey Satellite (TESS).
*4: Transit is a phenomenon caused by a planet partially blocking starlight as it passes in front of the star.
*5: MuSCAT series are multi-color cameras mounted on 1~2m class grand-based telescopes.
*6: The gravitational pull of a planet causes its parent star to wobble. The radial velocity method (or the Doppler method) uses the apparent variations in the velocity of a star in the direction of the line of sight to detect an unseen planet.
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See the full article here.
Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct.
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The University of Tokyo [(東京大学](JP) aims to be a world-class platform for research and education, contributing to human knowledge in partnership with other leading global universities. The University of Tokyo aims to nurture global leaders with a strong sense of public responsibility and a pioneering spirit, possessing both deep specialism and broad knowledge. The University of Tokyo aims to expand the boundaries of human knowledge in partnership with society. Details about how the University is carrying out this mission can be found in the University of Tokyo Charter and the Action Plans.
The university has ten faculties, 15 graduate schools and enrolls about 30,000 students, 2,100 of whom are international students. Its five campuses are in Hongō, Komaba, Kashiwa, Shirokane and Nakano. It is among the top echelon of the select Japanese universities assigned additional funding under the MEXT’s Top Global University Project to enhance Japan’s global educational competitiveness.
University of Tokyo is considered to be the most selective and prestigious university in Japan and is counted as one of the best universities in the world.
University of Tokyo’s alumni, faculty members and researchers include Prime Ministers, Nobel Prize laureates, Pritzker Prize laureates, astronauts, and Fields Medalists.
The university was chartered by the Meiji government in 1877 under its current name by amalgamating older government schools for medicine, various traditional scholars and modern learning. It was renamed “the Imperial University [帝國大學]” in 1886, and then Tokyo Imperial University [東京帝國大學]] in 1897 when the Imperial University system was created. In September 1923, an earthquake and the following fires destroyed about 700,000 volumes of the Imperial University Library. The books lost included the Hoshino Library [星野文庫], a collection of about 10,000 books. The books were the former possessions of Hoshino Hisashi before becoming part of the library of the university and were mainly about Chinese philosophy and history.
In 1947 after Japan’s defeat in World War II it re-assumed its original name. With the start of the new university system in 1949, Todai swallowed up the former First Higher School (today’s Komaba campus) and the former Tokyo Higher School, which thenceforth assumed the duty of teaching first- and second-year undergraduates, while the faculties on Hongo main campus took care of third- and fourth-year students.
Although the university was founded during the Meiji period, it has earlier roots in the Astronomy Agency [天文方] 1684), Shoheizaka Study Office [昌平坂学問所] 1797), and the Western Books Translation Agency [蕃書和解御用] 1811). These institutions were government offices established by the Tokugawa shogunate [徳川幕府] (1603–1867), and played an important role in the importation and translation of books from Europe.
In the fall of 2012 and for the first time, the University of Tokyo started two undergraduate programs entirely taught in English and geared toward international students—Programs in English at Komaba (PEAK)—the International Program on Japan in East Asia and the International Program on Environmental Sciences. In 2014, the School of Science at the University of Tokyo introduced an all-English undergraduate transfer program called Global Science Course (GSC).
Research
The University of Tokyo is considered a top research institution of Japan. It receives the largest amount of national grants for research institutions, Grants-in-Aid for Scientific Research. This massive financial investment from the Japanese government directly affects Todai’s research outcomes. According to Thomson Reuters, Todai is the best research university in Japan. Its research excellence is especially distinctive in Physics; Biology & Biochemistry; Pharmacology & Toxicology; Materials Science; Chemistry and Immunology.
In another ranking, Nikkei Shimbun surveyed about the research standards in Engineering studies based on Thomson Reuters, Grants in Aid for Scientific Research and questionnaires to heads of 93 leading Japanese Research Centers. Todai placed very highly in research planning ability, informative ability of research outcome and ability of business-academia collaboration in this ranking. Weekly Diamond also reported that Todai has very high research standards in terms of research fundings per researcher in COE Program. In the same article, it is also ranked very highly in terms of the quality of education by GP funds per student.
Todai also has been recognized for its research in the social sciences and humanities. Repec ranked Todai’s Economics department as very high in economics research. Todai has produced presidents of the Japanese Economic Association. Asahi Shimbun summarized the number of academic papers in Japanese major legal journals by university, and Todai has ranked very highly.
Research institutes
Institute of Medical Science
Earthquake Research Institute
Institute of Advanced Studies on Asia
Institute of Social Science
Institute of Industrial Science
Historiographical Institute
Institute of Molecular and Cellular Biosciences
Institute for Cosmic Ray Research
Institute for Solid State Physics
Atmosphere and Ocean Research Institute
Research Center for Advanced Science and Technology
The University’s School of Science and the Earthquake Research Institute are both represented on the national Coordinating Committee for Earthquake Prediction.
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